The first formulation (Formula of Universal Law).

I recently read the "Grundlegung zur Metaphysik der Sitten", Kants first book about his ethics. In my opinion it's very confusing. He doesn't make clear what's the role of "acting by duty". The first chapter suggests that duty shall be the only motivation of our acting. But in the second chapter he describes the use of the categorical imperative just as a test procedure for decisions in situations you're in due to already made [irrational] decisions. Additionally Kant doesn't really explain the connection between the general formulation and the "law of nature" formulation resp. he stays very notional.